(1.) Field of the Invention
The present invention relates to an aromatic polyester capable of forming an anisotropic melt. More particularly, it relates to an aromatic polyester composed wholly of aromatic constitutional units, which has a good melt-shapability and easily provides a shaped article such as fiber and film excellent in the physical properties.
(2.) Description of the Prior Art
Fibers having a relatively high tensile strength or high Young's modulus, for example, polyamide fibers such as polyhexamethylene adipamide fibers, polyester fibers such as polyethylene terephthalate fibers and inorganic fibers such as steel fibers or glass fibers, have been used as reinforcing fibers for organic polymeric materials such as rubbers and plastics according to uses of these organic polymeric materials. With recent progresses of technics in various industrial fields and increasing desires to attain energy-saving effects because of a fear of precarious supply of resources, it has been eagerly desired to improve the performances and reduce the weights in organic polymeric materials and also to develop high-performance organic polymeric materials that can be used as metal substitutes. In order to satisfy these requirements, it is necessary to develop reinforcing fibers excellent in the properties, especially the mechanical and thermal properties. However, conventional reinforcing fibers are insufficient in the mechanical properties, particularly the tensile strength and Young's modulus. Accordingly, various methods have heretofore been proposed for increasing the physical properties including these mechanical properties. However, no substantial improvements of the physical properties can be expected.
Aramide fibers such as poly-p-phenylene terephthalamide fibers and carbon fibers, which are known as high-performance fibers, have good performances and some of them are put into practical use. However, since the preparation process is complicated and these high-performance fibers are expensive, their application fields are limited and they are used only for special purposes.
It is known that fibers having a high Young's modulus can be obtained by ordinary melt spinning from a polyester capable of forming an anisotropic melt, which is called "liquid crystal polyester", and high-strength fibers can be obtained by heat-treating these fibers. It is expected that these fibers can be utilized as reinforcing fibers having good mechanical properties. Polyesters forming an anisotropic melt are characterized in that because of liquid crystal orientation characteristics in the molten state, the polyesters are highly oriented only by spinning without drawing and fibers having a high Young's modulus are obtained, and that if as-spun fibers are heat-treated at a high temperature close to the softening point in an inert gas atmosphere for a relatively long time, for example, for several hours to scores of hours, fibers having a tensile strength higher than 15 g/d can be obtained (see U.S. Pat. Nos. 3,991,013, 3,991,014, 4,118,372, 4,161,470, 4,238,559, 4,256,624 and 4,269,965).
It is considered that among polyesters forming an anisotropic melt, aromatic polyesters having a main chain composed solely of p-oriented benzene rings and ester bonds are especially preferred in the heat resistance and mechanical properties. However, shaping of these aromatic polyesters is generally difficult because they have a high melting point. For example, although a shapable wholly aromatic polyester formed from phenylhydroquinone and terephthalic acid has been proposed (U.S. Pat. No. 4,159,365), this polyester has a high melting point and the shapability is not sufficiently satisfactory.
Ordinarily, in order to obtain fibers having a high tensile strength, it is necessary to heat-treat spun fibers for a long time, and hence, actual industrial working is difficult.